This invention relates generally to a liquid crystal display device having a large number of pixels that is used for the display panel of measuring instruments, the instrumental panel of cars, personal computer image displays, television receivers, and so forth.
Liquid crystal display devices have been put into practical application as a kind of display which is compact in scale and light in weight and consumes less power. Among the liquid crystal display devices of this kind, an active matrix liquid crystal display device using thin film transistors having three terminals or MOS transistors formed on a single crystal silicon and an active matrix liquid crystal display device using non-linear resistance elements having two terminals and connected in series with each liquid crystal pixel have drawn an increasing attention in recent years view of display capacity.
In comparison with the active matrix having 3-terminal devices, the active matrix having 2-terminal devices has a smaller number of films to be formed and a smaller number of photoetching steps to be conducted and needs relatively rough patterning accuracy. Therefore, this type can be applied to a low cost display device having a large display area.
The following systems are known for the active matrix liquid crystal display having 2-terminal devices.
(1) varistor system PA1 (2) metal-insulating film-metal (MIM) system PA1 (3) diode system PA1 Fine etching step below 6 .mu.m is required, and a display having a size of above A4 becomes difficult to produce. PA1 The number of photoetching processes becomes at least 3, and the probability of occurrence of defects becomes higher as much. In addition, the production cost becomes also higher due to the increase in the number of production steps. PA1 Since the non-linear resistance film is below 600 .ANG. thick, MIM likely to be broken at the time of rubbing in the liquid crystal orientation treatment. PA1 The metal electrodes constituting MIM must have an electrically symmetric structure, and to this end, the same metal must be laminated and hence the number of production steps increases. PA1 When diodes are used as the non-linear resistance elements, at least two diodes must be disposed either parallel or in series for each pixel in order to obtain the same voltage-v-current characteristics in both forward and backward directions. Therefore, the number of the production steps and the ratio of occurrence of defects increase. PA1 Varistors using ZnO requires a thick film and a high sintering temperature. Therefore, it becomes difficult to make flat the substrate surface and the number of production steps increase. PA1 I: current PA1 K: constant PA1 .alpha.: non-linear coefficient PA1 E: field intensity 10.sup.6 V/cm
The varistor system (1) and the MIM system (2) are disclosed in Japanese Patent Laid-Open No. 105285/1980 and Japanese Patent Laid-Open No. 161273/1980, respectively. The operation system of the active matrix liquid crystal display device having the 2-terminal devices will be described with reference to the MIM system by way of example.
FIG. 9 of the accompanying drawings is a longitudinal sectional view of a heretofore known liquid crystal display device using the MIM type non-linear resistance elements of the system (2). The drawing illustrates only one pixel. FIG. 10 is a circuit diagram of a liquid crystal panel having a large number of row and column electrodes using the heretofore known non-linear resistance elements.
Referring to FIG. 9, reference numerals 90 and 91 represent upper and lower transparent substrates, 92 is a liquid crystal layer, 93 is a display transparent electrode for the upper substrate and 94 is a metallic tantalum electrode for the lower substrate.
The transparent electrode 93 and the metallic tantalum electrode 94 together form row and column electrodes, each consisting of more than 100 number of electrodes. Reference numeral 95 represents a display pixel electrode and reference numeral 96 is an insulating film formed by anodic oxidation of the metallic tantalum electrode 94. MIM consists of the electrode 94, the insulating film 96 and the electrode 95.
FIG. 10 is an equivalent circuit diagram of a liquid crystal display device having the sectional structure shown in FIG. 9. Reference numeral 100 represents the row electrode groups and 101 represents the column electrode groups. A liquid crystal 102 and a non-linear resistance element 103 connected in series to each other are formed at the point of intersection of each row electrode and each column electrode.
The principle of operation of the liquid crystal panel of this kind can be understood as follows. An ordinary simple matrix drive method is such that a large number of row electrodes 100 shown in FIG. 10 are selected line by line from the upper line, and the data is written by the column electrodes 101 during this selection period. The voltage levels to be applied to the row and column electrodes are determined by a system which is generally referred to as a "voltage averaging method". This method is applied to the panel having the non-linear resistance elements shown in FIG. 10, and a bias method of from 1/3 to 1/15 is employed.
FIG. 11a is an equivalent circuit diagram of one known pixel, and a non-linear resistance element and a liquid crystal are shown connected in series with each other. Symbol C.sub.LC represents the capacity of the liquid crystal, R.sub.LC is the resistance thereof, and C.sub.I is the capacity of the non-linear resistance element and R.sub.I is the resistance thereof. The resistance R.sub.I is a function of a voltage. FIGS. 11b and 11c show the waveforms of voltages applied to the pixel when it is turned on and the waveforms are heretofore known. Solid line represents the waveform of the voltage impressed upon the pixel, and this voltage is practically applied between the point A and the point C in FIG. 11a. Broken line represents the waveform of the voltage at the point B in FIG. 11a. Therefore, the hatched portions in FIGS. 11b and 11c represent the effective voltage applied to the liquid crystal.
In order to obtain a liquid crystal display operation having sufficient contrast, the effective voltage applied to the liquid crystal at the time of turn-on must be greater than the sum voltage V.sub.SAT of the liquid crystal, the effective voltage applied to the liquid crystal at the time of turnoff must be lower than the threshold voltage V.sub.TH of the liquid crystal and at the same time, the value of the resistance R.sub.I in the non-selection period is at least equal to the value of the resistance of the liquid crystal. In other words, the time constant .tau..sub.1 for writing or electrical charging at the time of turn-on, the time constant .tau..sub.2 for holding the electric charge and the time constant .tau..sub.3 for inhibiting the write can be expressed by the following equation (1) through (3). Here, the capacity of the liquid crystal is assumed to be 5.times.10.sup.-13 F. EQU 5.times.10.sup.-6 &lt;.tau..sub.1 &lt;1.times.10.sup.-4 ( 1) EQU 1.6.times.10.sup.-3 &lt;.tau..sub.2 ( 2) EQU .tau..sub.3 &gt;1.times.10.sup.-4 ( 3)
where ##EQU1##
Therefore, EQU 10.sup.7 &lt;R.sub.I (V.sub.ON)&lt;2.times.10.sup.8 ( 4) EQU 3.times.10.sup.9 &lt;R.sub.I (V.sub.NON)R.sub.LC /[R.sub.I (V.sub.NON)+R.sub.LC ] (5) EQU R.sub.i (V.sub.OFF)&gt;2.times.10.sup.8 ( 6)
Generally, the relation C.sub.I &lt;C.sub.LC /5 must be satisfied in order to apply a sufficient voltage to the non-linear resistance element where C.sub.I represents the capacity of the non-linear resistance element.
In the conventional MIM system using the insulating film for the non-linear resistance element, the insulating film 96 shown in FIG. 9 must have a thickness from 100 to 600 .ANG..
From the restrictive condition of the capacity ratio C.sub.I .ltoreq.C.sub.LC /5 between the non-linear resistance element and the liquid crystal, the overlap area of the non-linear resistance element with the upper and lower electrodes must be at most about 6 .mu.m.times.6 .mu.m, and the density of a current flowing through this portion is at least 5 A/cm.sup.2 from equation (4).
In the liquid crystal display of this kind wherein the insulating film 96 has the thickness from 100 to 600 .ANG., the non-linear resistance element is likely to be broken by the mechanical load that is generated by liquid crystal orientation rubbing treatment. If the materials of the upper and lower substrates constituting the non-linear resistance element are different, non-linearity of the voltage-current characteristics becomes asymmetric with respect to the positive and negative polarity of the impressed voltage due to the difference of the potential barrier between the respective electrodes and the insulating film, so that the liquid crystal panel undergoes rapid degradation if the electro-chemical reaction develops on the interface between the liquid crystal layer and the electrodes. In order to prevent this problem, the electrodes made of the same material must be used, so that the number of photoetching steps increases, the number of photomasks to be used is at least 4 and eventually, the production cost of the panel becomes higher.
This also holds true of the liquid crystal display device using diodes as the non-linear resistance elements. When varistors having a composition ZnO is used, ZnO must be at least 25 .mu.m thick and the sintering temperature during its production process must be at least 500.degree. C. In addition, an etching step of an extremely thick film is necessary, and a driving voltage must be as high as at least 30 V.
The problems of the prior art described above can be summarized as follows.